The SIR provided 74% K 15 N03 containing 87.6g of isotope; 803.9g 17.5% K"N03 contining 131g of isotope; 4978.9 g 17.5% K 15 NO containing 10.4g of isotope; 4 lOg 21% of K 15 N63 continaing 186g of isotope; 2030. 1 g Improved understanding of the nitrogen cycle in estuaries is needed to help explain how anthropogenic changes in nitrogen inputs are affecting coastal systems. We propose to experimentally test conceptual and numerical models of nitrogen dynamics in an estuary by adding 15N-enriched dissolved inorganic nitrogen to the upper reaches of the Parker River-Plum Island Sound estuary, the site of an ongoing NSF-funded Land Margin Ecosystem Research (LMER) program. Previous whole-system labeling experiments in forested watersheds, lakes and rivers have demonstrated the feasibility, power and cost effectiveness of the tracer addition approach. In the Parker River-Plum Island Sound estuary, intensive measurements of the distribution of the added nitrogen tracer will yield estimates of fluxes and residence times of nitrogen in the water, food web components and sediments. The 15N addition experiment will provide information to test and refine complex models of estuarine N flow that are otherwise very difficult to validate. The product will be greatly improved understanding of the transport, transformation and fate of nitrogen in riverine saltmarsh estuaries.